On 10/2/2023 8:31 am, Sean Christopherson wrote:
Now that KVM disallows changing feature MSRs, i.e. PERF_CAPABILITIES,
after running a vCPU, WARN and bug the VM if the PMU is refreshed after
the vCPU has run.
Note, KVM has disallowed CPUID updates after running a vCPU since commit
feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN"), i.e.
PERF_CAPABILITIES was the only remaining way to trigger a PMU refresh
after KVM_RUN.
A malicious user space could have saved the vcpu state and then deleted and recreated a new vcpu w/ previous state so that it would have a chance to re-set the features msr. The key to this issue may be focused on the KVM_CREATE_VM interface. How about the contract that when the first vcpu is created and "after KVM_RUN of any vcpu", the values of all feature msrs for all vcpus on the same guest cannot be changed, even if the (likely) first ever ran vcpu is deleted ?
Cc: Like Xu <like.xu.linux@xxxxxxxxx> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- arch/x86/kvm/pmu.c | 3 +++ arch/x86/kvm/x86.c | 10 +++++++++- 2 files changed, 12 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c index 612e6c70ce2e..7e974c4e61b0 100644 --- a/arch/x86/kvm/pmu.c +++ b/arch/x86/kvm/pmu.c @@ -589,6 +589,9 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info) */ void kvm_pmu_refresh(struct kvm_vcpu *vcpu) { + if (KVM_BUG_ON(kvm_vcpu_has_run(vcpu), vcpu->kvm)) + return; + static_call(kvm_x86_pmu_refresh)(vcpu); }diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.cindex 186cb6a81643..1b14632a94a3 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3626,9 +3626,17 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) if (data & ~kvm_caps.supported_perf_cap) return 1;+ /*+ * Note, this is not just a performance optimization! KVM + * disallows changing feature MSRs after the vCPU has run; PMU + * refresh will bug the VM if called after the vCPU has run. + */ + if (vcpu->arch.perf_capabilities == data) + break; + vcpu->arch.perf_capabilities = data; kvm_pmu_refresh(vcpu); - return 0; + break; case MSR_EFER: return set_efer(vcpu, msr_info); case MSR_K7_HWCR:
